The present invention relates to a biaxially stretch-oriented and heat-set polyester film which on at least one surface is provided with a coating in which a copolyester that includes sulfonate groups is contained.
The manufacture of copolyesters containing sulfonate groups and their use as constituents of coatings for plastic films are known (U.S. Pat. No. 3,563,942). They can be applied in the form of aqueous solutions or dispersions and are employed for layer supports or adhesion-promoting intermediate layers (German Patent No. 28 13 014). The copolyester coatings per se do not have an antistatic effect. They may be applied to the finished plastic film or during film manufacture (U.S. Pat. No. 4,476,189). Apart from their inferior adhesion properties, polyester films have the disadvantage that, despite their frequent use and wide field of applications, they charge up electrostatically to an undesirably high degree, like all plastics do.
In order to avoid this disadvantage it is known to add antistatic agents to the plastic materials or to provide them with a thin surface coating of an antistatic agent. When materials are treated according to the first-mentioned method, the antistatic effect which can be achieved is unsatisfactory or, if the added amount of antistatic agent is increased, their mechanical properties become clearly worse. When the materials are provided with superficial coatings, a more efficient prevention of electrostatic surface charge-up is possible. In most cases, the antistatic agents used are water-soluble, low molecular-weight or polymeric products. Thus, the applied coatings only possess a limited resistance to moisture or water, respectively. Moreover, these coatings are easily abraded mechanically, whereby their antistatic action is lost, or they get tacky when the films are heated.
If a combination of customary antistatic agents and binders is used, antistatic coatings exhibiting improved mechanical strength are obtained, but their antistatic effect is lost when water is allowed to act upon the film.